While a sintered Si.sub.3 N.sub.4 has been known as a ceramic material excellent in high temperature strength, it has a disadvantageously low oxidation resistance. On the other hand, silicon oxynitride (Si.sub.2 N.sub.2 O), which has been known as a compound excellent in oxidation resistance, has inferior sintering properties compared to Si.sub.3 N.sub.4, and is scarcely compacted. For example, the density of a sintered silicon oxynitride, which is obtained by adding 5% by weight of Al.sub.2 O.sub.3 or Y.sub.2 O.sub.3 to the silicon oxynitride and sintering under a gas pressure (as described in "Sintering of Silicon Oxynitride", lecture abstract, 25th Ceramic Basic Symposium, 1987, p. 18), is only at most 2.5 g/cm.sup.3, 85% of the theoretical density, resulting in insufficient strength. However as Lewis carried out when a large amount of Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 and the like, as a sintering assistant is added (Materials Science and Engineering, Vol. 71 (1985), pp. 87-94), a compacted product is obtained but its high temperature strength and oxidation resistance are reduced. For these reasons improvements sintered silicon oxynitrides are desired.